Slump test is used to measure consistency of fresh concrete mix. Consistency and workability are often confused with one another. Consistency is degree of wetness of concrete. Or you can say how much wet the concrete is. Workability means the ease with which concrete can be moved and placed in forms without segregation. Consistency cannot measure workability but it can give indication of workability of concrete. Consistency of concrete is measured with the help of slump test.
ASTM C143/143M (Standard Test Method for Slump of Hydraulic Cement Concrete)
- Slump cone [ 100 mm (4 in) upper dia, 200 mm (8 in) lower dia and 300 mm (12 in) height ]
- Tamping rod [ 600 mm (24 in) high, 15 mm (5/8 in) dia and hemispherical tip at both ends ]
- Base plate or tray [ 600 mm x 600 mm (24 in x 24 in) ]
- Concrete scoop, brush and steel ruler
- Clean the inside of the slump cone with brush and damp it with wet cloth
- Place this cone (small dia upwards) on a smooth, horizontal and rigid base plate or tray
- Held firmly the slump cone in place with two feet resting on two cone’s foot pieces
- Fill this cone with fresh concrete in three equal layers by volume
- Tamp each layer with 25 strokes of the tamping rod, while uniformly distributing the strokes over the cross-section of the layer. When tamping the first (bottom most) layer, the rod should be inclined slightly, and about half of the 25 strokes should spiral towards the center. Each layer should be tamped to its full depth, allowing the rod to penetrate through into the layer below.
- After the top layer has been tamped the concrete should be struck off level with the top of the cone by a sawing and rolling motion of the tamping rod. Any spillage is cleaned away from around the base of the cone.
- Slowly lift the cone vertically from the concrete. The concrete will fall due to its self weight.
- If it collapses or shears off laterally, the test should be repeated with another sample of the same concrete and the type of slump noted. If two consecutive tests shows falling away or shearing off of a portion of the concrete, the concrete probably lacks necessary plasticity and cohesiveness for the slump test to be applicable.
- Place the empty cone beside the concrete upside down. Place tamping rod above the cone and measure the difference between the height of the cone and of the highest point of the concrete being tested with help of a steel ruler.
- The slump should be recorded to the nearest 5 mm (1/4 in)
Types of Slump
There are three types of slump:
- True slump
- Shear slump
- Collapse slump
When the cone is removed, the slump may take one of three forms. In a true slump the concrete simply subsides, keeping more or less to shape. In a shear slump the top portion of the concrete shears off and slips sideways. In a collapse slump the concrete collapses completely. Only a true slump is of any use in the test. If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated. If two consecutive tests shows falling away or shearing off of a portion of the concrete, the concrete probably lacks necessary plasticity and cohesiveness for the slump test to be applicable. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which the flow table test is more appropriate.
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The slump test is the most simple workability test for concrete, involves low cost and provides immediate results.